Development of portable electric and electronic devices such as, typically, personal computers and handy-phones in recent years has provoked a phenomenal increase of demand for batteries used as power source of these devices. Especially lithium batteries have been studied intensively in expectation of obtaining a battery capable of providing a high energy density as lithium has small atomic weight and high ionization energy, and such lithium batteries have now come to be used popularly as a power source of the various portable electric and electronic devices and for various other purposes.
On the other hand, with such prevalence of lithium batteries, increasing concern has been shown recently on safety of the batteries in practical use thereof, in view of the enlargement of inherent energy due to the increasing amount of the active material contained in the battery and the increase of the content of the organic solvent which is an inflammable material used for the electrolyte. Use of a solid electrolyte, which is a nonflammable material, in place of the conventional organic solvent electrolytes is very effective for securing safety of the lithium batteries, and the development of a solid-state lithium battery with high safety feature has been required.
For obtaining a high-voltage battery, a specific material such as lithium cobalt oxide (Li.sub.1-x CoO.sub.2) is used as the active material for the positive electrode of a lithium battery. This material is of a metastable phase that can be formed as a result of extraction of lithium ions from LiCoO.sub.2, which is a high temperature stable phase. LiCoO.sub.2 has a structure in which the respective triangular lattices of oxygen, lithium and cobalt are accumulated in the order of O--Li--O--Co--O--Li--O, with lithium ions present between the CoO.sub.2 layers. Said material can serve as an electrode material of a lithium battery as reversible insertion and extraction of lithium ions take place between said layers.
The lithium ions in LiCoO.sub.2 play the role of having the CoO.sub.2 layers attracted to each other by virtue of electrostatic attraction between the cationic lithium atoms and anionic oxygen atoms. When the lithium ions are extracted from LiCoO.sub.2, since there no longer exists Li in the O--Li--O structure, electrostatic repulsive force between the oxygen atoms in the CoO.sub.2 layers elevates to cause an interlaminar stretch. Consequently, there takes place expansion or shrinkage of the crystal lattices due to the lithium ion insertion/extraction reactions during charging or discharging of the lithium battery.
The interface between the electrode active material and the electrolyte in a solid-state battery using a solid electrolyte is a solid/solid interface which, as compared with the solid/liquid interface in the conventional liquid electrolyte batteries, has greater difficulties in enlarging the contact area between the electrode active material and the electrolyte, namely the electrochemical reaction interface. Further, in case a material which undergoes a volumetric change during charging or discharging, such as the afore-mentioned lithium cobalt oxides, is used as the electrode active material, it is difficult to keep a steady interface between such electrode active material and the solid electrolyte. Consequently, the interface is always subject to change during operation of the battery, and the change of the interface causes a corresponding change of overvoltage in the electrode reaction.
Charging of a battery can be effected by either a constant-current charging method or a constant-voltage charging method. In the case of the constant-current charging method, charging is terminated when the terminal voltage of the battery has reached a certain value, so that it is necessary to set the charging voltage of the battery no matter which method is used.
However, as explained above, the overvoltage of the reaction changes incessantly at the electrode of a solid-state battery, so that the electrode potential on during charging is not constant even in case the terminal voltage of the battery is kept constant during charging. This means that the battery may be charged deeply in case the reaction overvoltage decreases even though charging is performed at a constant voltage.
Li.sub.1-x CoO.sub.2 formed by extraction of lithium ions from LiCoO.sub.2 shows a high equilibrium potential of 4 V Vs Li or above, but the crystal structure becomes unstable due to the afore-mentioned repulsion force between the oxygen atoms. Therefore, in order to effect stabilized operation of a battery using said material as the electrode active material, it is necessary to limit the maximum amount of extraction lithium ions within a reversible range. Excessive elimination of lithium ions causes a change of crystal structure, making it unable to show the reversible insertion/extraction reactions of lithium ions. Therefore, in a charge and discharge cycle of a solid-state lithium secondary battery using said material as the electrode active material, when there takes place a decrease in overvoltage such as mentioned above and the battery is charged deeply, extraction amount of lithium ions exceeds the limit of reversible range, resulting in deterioration of the insertion/extraction reactions of lithium ions between the crystal layers. This leads to a decrease of battery capacity with the charge and discharge cycle of the battery, giving rise to the problem of reduced charge and discharge cycle life of the battery.
Said phenomenon of deep charging of the battery is also caused by inaccuracy of charging control of the charger or drift of charging voltage with time. Therefore, precise charging control is essential for elongating the cycle life of the battery. This is, however, hardly achievable by use of an uncostly charger, giving rise to the problem of necessity of using a costly charger.
The foregoing explanation concerns the case where LiCoO.sub.2 was used as the positive electrode active material, but similar problems may arise even in the case of a solid-state lithium secondary battery using other materials such as Li.sub.1-x NiO.sub.2 and Li.sub.1-x Mn.sub.2 O.sub.4 as the positive electrode active material since these materials also show an equilibrium potential exceeding 4 V when taking a metastable phase.
There are other factors, such as volumetric change of the electrode active material consequent to charging or discharging, which greatly affect the cycle characteristics of a solid-state battery. Generally, a solid-state battery is composed of solid particles, and these solid particles become plastic on occurrence of displacements such as expansion or contraction in case no force is acting in the direction of causing aggregation of the particles. Therefore, in a non-pressurized solid-state battery, there are formed the voids among the consisting solid particles therearound due to expansion or contraction of the electrode active material caused by charging or discharging. This is causative of a decrease of the electrochemical reaction area after repetition of charge and discharge resulting in a reduction of current collectability of the electrode active material and degradation of battery performance.
The present invention is envisaged to solve the above problems and provide a solid-state lithium secondary battery with excellent charge and discharge cycle characteristics, a battery assembly, and a method for charging them.